With power lines now extended to all six reactors, work to
restore core cooling and other electrical systems at the Fukushima Daiichi
nuclear plant continued Wednesday, interrupted for a time by black smoke coming
from unit 3.
Workers evacuated at 4:20 p.m. Japan time when smoke similar
to that seen from unit 3 on Monday was spotted. It thinned out after an hour,
Kyodo News quoted plant owner Tokyo Electric Power Co. as saying. Japan’s Nuclear
and Industrial Safety Agency indicated radiation levels were unchanged
following the plume’s appearance, and work eventually continued.
At unit 1, workers increased the amount of seawater they’ve
been injecting into the core to nine times the previous volume Wednesday as
the pressure vessel’s exterior temperature topped proscribed
design limits. NISA reported exterior temperatures exceeded 400 C before
dropping to 350 C by noon, according to Kyodo. Unit 3’s temperature also was a
concern, reported by NISA at 304.8 C. A Kyodo report said the maximum vessel
temperature intended by the BWR’s designers is 302 C.
The work to test and repair cooling equipment has proven difficult
and potentially dangerous, with Reuters quoting the IAEA as saying that 18
workers have been exposed to radiation since the accident. None required
medical treatment, although one worker received a high dose of about 0.1
sieverts. Kyodo, quoting TEPCO officials, also indicated two workers were taken
to a hospital Wednesday for injuries sustained while working on power systems
at the plant, but they were not exposed to radiation.
Also Wednesday, NISA corrected an earlier report of radiation
readings of 500 millisieverts per hour at the plant. The reading was 500
microsieverts.
Outside the plant, radiation levels remain low, but concerns
persist regarding contaminants presumably from the plant detected in nearby
crops. On Wednesday, testing at three Tokyo water plants also detected iodine
131 at levels considered unsafe for infants.
The source of the radiation – whether from overheated
spent-fuel tanks, a suspected containment breach in unit 2 or regular
containment venting as crews use sea water to control reactor temperatures
– remains unknown. Over the two last
weeks, three reactors are suspected to have sustained fuel damage and suffered
hydrogen explosions, and a fire burned the exterior building of reactor 4.
Reports now indicate the March 11 earthquake-triggered tsunami that took out
backup generators at the plant was more than 40 feet high.